How to properly import from CATIA v5 to Blender?
You can use a Decimate modifier in Blender to make your mesh more manageable. First, import your high-res .stl in to Blender. To try and minimize shading artifacts and improve the Decimate results, select the whole mesh in Edit Mode and apply Remove Doubles (Mesh > Clean up > Remove Doubles). 

After that, go to the modifiers panel and change the method to Planar (I can't tell what exactly your mesh is from the pictures you gave, but It looks like it is mostly flat).

Adjust the Angle Limit and Delimit settings so that your mesh looks good without a loss of detail on high-res areas, then apply the modifier.